Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received February 25, 2017; final manuscript received September 22, 2017; published online December 4, 2017. Assoc. Editor: Matevz Dular.

Abstract

Savonius vertical axis wind turbines (VAWTs) typically suffer from low efficiency due to detrimental drag production during one half of the rotational cycle. The present study examines a stator assembly created with the objective of trapping cylindrical flow for application in a Savonius VAWT. While stator assemblies have been studied in situ around Savonius rotors in the past, they have never been isolated from the rotor to determine the physics of the flow field, raising the likelihood that a moving rotor could cover up deficiencies attributable to the stator design. The flow field created by a stator assembly, sans rotor, is studied computationally using three-dimensional (3D) numerical simulations in the commercial computational fluid dynamics (CFD) package Star-CCM+. Examination of the velocity and pressure contours at the central stator plane shows that the maximum induced velocity exceeded the freestream velocity by 65%. However, flow is not sufficiently trapped in the stator assembly, with excess leakage occurring between the stator blades due to adverse pressure gradients and momentum loss from induced vorticity. A parametric study was conducted on the effect of the number of stator blades with simulations conducted with 6, 12, and 24 blades. Reducing the blade number resulted in a reduction in the cohesiveness of the internal swirling flow structure and increased the leakage of flow through the stator. Two unique energy loss mechanisms have been identified with both caused by adverse pressure gradients induced by the stator.

Contours of normalized tangential velocity inside the stator at the central plane with velocity vectors overlaid on (d) as representative of the shape of the flow field. Flow is from left to right and is normalized by freestream velocity (V∞). Re = (a) 278,311, (b) 347,888, (c) 417,466, and (d) 466,171. Reynolds number (Re) is defined based on external diameter of the stator and freestream velocity (V∞).

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